The present invention relates to a multi-color image display device that is capable of reconciling wide range color reproduction and high-definition display.
A liquid crystal display device, which represents an example of conventional image display devices, is provided with a white light source or a tricolor light source, having a maximum value of three colors of red, green and blue, and a subpixel that is disposed for each of the pixels for selectively transmitting a color by way of color filters of red, green and blue. The liquid crystal display device displays an image by applying an electric field to a liquid crystal enclosed between electrodes that form each of the subpixels, which electrodes are supplied with a voltage in accordance with image information, so as to control the transmittance or reflectance of colors.
The range of expression realized by the above-described system is limited to a range inside a triangle formed by the tricolor light source on a chromaticity diagram. Therefore, it is impossible for the system to reproduce all colors existing in nature, and the system sometimes cannot meet the demands of displaying a color tone, texture, brilliance, etc. that should appeal to the human senses. For example, objectives that are expected to be accomplished in terms of an insufficient range of expression include a higher level of high-fidelity image reproduction, such as diagnostic precision in the field of telemedicine that employs a communication network, and the expression of values of curios and merchandise in electronic museums and electronic transactions. Hence, various multi-color display devices have been proposed in order to meet such demands.
For example, in a natural vision system proposed by Japanese Patent Laid-open No. 7-330564 and a Technical Report No. EID2000-228 (2000-11) issued from Institute of Electronics, Information and Communication Engineers, a color is no longer picked up and displayed by way of the three primary colors, but is treated as spectrum information to be picked up, converted, transmitted and displayed as multi-color data. In this system, a multi-color camera of 16 bands is used as a picking up system to measure information regarding illumination for an object and to transmit the measured information together with other data, thereby realizing a transmission and reproduction of high-fidelity image data between remote locations.
Also, in order to meet the above demands, there has been developed a six primary color display device wherein projection images respectively captured by two liquid crystal projectors are synthesized. In the six primary color display device, narrow bandwidth color filters of three primary colors having different transmission wavelength bandwidths, respectively, are disposed in light paths of red, green and blue in each of the optical systems of the projectors, to thereby improve the color purity, and a six primary colors display is realized by combining two types of projectors having different color reproduction ranges.
There have been proposed other display systems, such as a time-division system wherein multi-color color filters are provided on a rotating disk to display colors on the basis of time-division, a spatial pixel arrangement system, a plane division system and a system combining these systems.
Characteristics of a multi-color display device will be explained in detail with reference to FIG. 11. FIG. 11 is a chromaticity diagram showing color reproduction ranges that are indicated by numerical values. A visible area 501 represents a range of colors capable of human perception, and a display device is required to display a range as wide as possible in the visible area 501 to achieve excellent color reproducibility. Characteristic 502 represents an example of the display range of the conventional three primary color display device, which is an area of a triangle formed by the three primary colors. In turn, a display area 503 of the multi-color display device is expanded by way of the multi-color display of four or more primary colors. The present example represents a display produced by way of six primary colors; and, therefore, the display area is considerably expanded as compared with the conventional three primary color display. In the case of three primary colors, the mixing ratio of red (R), green (G) and blue (B) for each of colors is uniquely defined; however, in the case of a six primary color display, the degree of freedom of display is increased and the mixing ratio is not defined uniquely. A color conversion method in the multi-color display is disclosed in Japanese Patent Laid-open No. 6-261332, for example. Thus, it is apparent from FIG. 11 that the multi-color display enables the production of a display that is high in the color purity of each of the primary colors, which was not achieved by the conventional three primary color display, as well as the reproduction of colors that are profoundly impressive for human sensitivity, such as deep red, deep blue and fresh green.
As mentioned above, it has been disclosed that the multi-color display device can reproduce a texture having the same quality as that captured by a sender without being influenced by the ambient light, by performing correction processing based on the spectral information of ambient light of both of the image pick-up location and image displaying location.
A multi-color display device that can display even a texture of an object is suitable for a large screen display employing a screen of the type which is used in electronic museums and theatres, and there are expected applications thereof related to a personal computer and a mobile information terminal that are improved in portability by the downsizing and lightening of these devices. Especially, for the field of portable display devices, a display device that can correct the influences of illumination and which has a wide display range is in demand, since the ambient illumination for the portable display device changes with movement. In order to clarify the problems in realizing a multi-color display device as a direct-view type liquid crystal display device feasible for downsizing and lightening, a description will be made of a color reproduction system employed in a conventional liquid crystal display device.
Examples of the color reproduction system for the conventional direct-view type liquid crystal display device include a subpixel system using a color filter and a color field sequential system using a tricolor flashing light source, not a color filter.
In a color filter system, a white light source for continuous lighting is used. An area for one pixel is divided into three subpixels, and the three subpixels are respectively provided with color filters of red, green and blue, as well as pixel electrodes. In the case of an active matrix, the system is further provided with an amorphous, a polycrystalline or a monocrystalline film transistor that is placed between a signal wiring and a pixel electrode, and which functions as a switching element for writing a voltage signal. When the brightness from the light source is constant, the brightness of the display device is determined by the transmittance of the color filters and the aperture ratio of a pixel, that is, a ratio of the area of the aperture. In the case of realizing a multi-color display device by way of the subpixel system using color filters, the aperture ratio may decrease due to an increase in the number of subpixels, if an area for one pixel is constant, while the resolution may decrease, if the area for one subpixel is constant. When color filters each having a narrow transmission bandwidth and a high color purity are used to increase the number of primary colors, the brightness may decrease due to a deterioration in the transmittance. In such cases, a strong light source will be required to improve the brightness, which leads to an increase in the power consumption and unnecessary heating.
In turn, in the conventional color field sequential system, which does not employ color filters nor a subpixel structure, three primary color light sources of red, green and blue, that can be switched on and off at a high speed, are lit in time sequence, and the transmittance of the pixels is controlled by applying signal voltages to liquid crystals of the pixels in synchronization with the lighting.
The color field sequential system is characterized by its capability for both high brightness and high-definition display owing to the elimination of the color filters and subpixels, although the system requires a liquid crystal display mode having high speed response properties and three primary color light sources. To realize a multi-color display device by way of the color field sequential system, it is necessary to provide a high speed liquid crystal display mode in accordance with an increase in the number of primary colors. For the conventional three primary color display, a response in 2 to 3 milliseconds is required, since it is necessary to respond within a period that is obtained by subtracting the time for writing voltages to pixels and the time for switching on a fluorescent lamp that is used for ordinary illumination.
In the case of applying the system to a multi-color display device of six primary colors, for example, the total time of a period required for writing voltages for one color, a period for the liquid crystal to respond and a period for illumination is about 2.8 milliseconds, with a display frequency being set at 60 Hz, that does not cause a flicker. In this case, the period for writing voltages to pixels and the switching period for illumination consume most of the response time, if the conventional driving system is employed; and, therefore, a response including half tones in not more than 1 millisecond will be required. Thus, it is difficult to apply the conventional color field sequential system to a multi-color display device.
Taking into consideration portable display devices, other than the liquid crystal display device, candidate systems may be a CRT (Cathode Ray Tube) of the type that is widely used for monitors, an EL (Electroluminescent Display) display device using organic or inorganic luminescent materials, a PDP (Plasma Display Panel) and so forth. Since these display systems are of the emission type, they reproduce colors by constructing subpixels in accordance with the number of primary colors to be used, and some printing techniques are applied to the construction of subpixels. Therefore, it is difficult to realize a multi-color display device using three primary colors, or more than three primary colors, with high definition sufficient to represent a texture in terms of the human sense.